1. Field of the Invention
This invention generally relates to a low calcium cementitious material mainly composed of low calcium fly ash which can be processed into low calcium cement at room temperature and a method of manufacturing the low calcium cement.
2. Description of the Prior Art
Fly ash is the by-product of a coal-fired boiler in a combined heat and power system. The global annual emission of fly ash is about 500 million tons. The annual emission of fly ash in China is approximately 160 million tons, wherein only 15% of the fly ash is recycled. Because a large surplus of fly ash is piled up on the ground or buried underground, large area of land is occupied and serious environmental pollution is caused.
Fly ash not only has great chemical stability, but also has many superior physical properties. Hence, fly ash is a generally accepted engineering material. However, because the low content of calcium oxide of class F fly ash (calcium oxide content<10%), conventional alkali-activated cement that mixes the class F fly ash with sodium silicate, sodium hydroxide (or potassium hydroxide), and water cannot harden at room temperature. The alkali-activated cement must be activated at high temperature for several hours to harden, wherein its crystalline phase is characterized to be zeolites (Na2Al2Si3O10) by the X-ray diffraction (XRD). Otherwise, fly ash, especially class F fly ash, should be mixed with metakaolin or calcium-containing materials such as, slag, gypsum for increasing the content of calcium to harden at room temperature.
On the other hand, ordinary portland cement is generally poor in acid-resistance and fire-resistance due to its high content of calcium. Therefore, such cement cannot be used as construction material in acidic or high temperature environment. For example, an ordinary pre-cast sewer pipe is generally coated with high alumina cement to increase its acid-resistance, thus increasing the material cost. Moreover, a collapse will occur at high temperature of about 400° C. when the cement suffers fire attack, resulting in a sudden strength break down of the cement.